A small-displacement two-cycle engine is generally used for a portable work machine such as a brushcutter, a chain saw and the like. Such a portable work machine often includes an automatic choke to improve startability of a cold engine. The automatic choke has various structures. For example, the automatic choke may include a solenoid valve integrated with a carburetor.
Such an automatic choke includes a starting fuel supply passage which extends from a fuel reservoir of the carburetor and which is apertured in an intake passage of the carburetor, and the solenoid valve as a flow passage opening and closing unit provided in the middle of the starting fuel supply passage. When the solenoid valve is opened, a fuel to be excessively enriched, i.e. a starting fuel, is supplied to the intake passage from the fuel reservoir through the starting fuel supply passage due to a negative pressure in the intake passage.
In the two-cycle engine having such an automatic choke, the intake passage of the engine in which a piston valve is employed as an intake method is apertured in a cylinder as an intake port, and is opened and closed by a reciprocating piston. In such a conventional two-cycle engine, when the intake port is opened by moving the piston from a bottom dead center to a top dead center, the negative pressure is applied in the intake passage from the crankcase to supply a fuel to the intake passage from the starting fuel supply passage (for example, see Patent Document 1).
[Patent Document] JP-A-2002-339805